For many purposes it is necessary to form seals or bonds between glass and metal. Examples of such purposes are light bulbs or vacuum tubes wherein metal wires or prongs must pass through glass envelopes in which vacuum or special atmosphere is maintained. Another example is the metal cap that seals the open end of a glass envelope such as a television picture tube. To be useful for such purposes, particularly when the seal must be sound over a temperature range, it is essential that the metal have very special characteristics.
Of primary importance is that the thermal expansion characteristics of the metal must be the same as, or very similar to, the thermal expansion characteristics of the glass. If there is a great difference in thermal expansion characteristics, the brittle glass envelope will shatter when temperature changes are experienced.
It is also important that the metal can be prepared with an adherent oxide coating that is compatible with glass. During the sealing process, the glass must be able to wet the oxide coating and bond to it, and it is also essential that the oxide coating to which the glass is bonded is sufficiently adherent to the metal so that it will not break away and so that a vacuum seal can be formed.
Alloys suitable for sealing to glass are known and one of the most widely used of these is an alloy containing from 38-45% nickel, usually about 42%; from 3-15% chromium, usually about 5.5%; and the remainder iron and residuals. Manganese is frequently employed in small amounts (typically 0.15-0.25%) to improve the melting characteristics of the alloy. When a composition is indicated as a percent herein, the percent by weight of the total composition is indicated.
Thermal expansion characteristics, the character of the glass coating, and particularly the grain growth characteristics of metals are sensitive to impurities. It is desirable to prepare alloys for bonding to glass by vacuum induction melting in order that extremely clean alloys can be prepared. Alloys thus produced are worked to final form employing both hot working and cold working steps. For example, many such alloys are subjected to a series of hot rolling and cold rolling steps with intermediate annealing steps, and the final grain pattern in such metal is unpredictable.
Particularly with large elements for bonding to glass, such as the anode caps for television tubes, large grains are prevalent and they produce an effect known as "orange peel" which describes the appearance of the metal surface. The large grains cause forming defects in parts which render them unacceptable for their intended uses.